AKL98

Hi, I'm building a 355 and I'm considering Speed Pro Flat top pistons offering 9:8:1 Compression at 64cc with the vortec heads I'm running. Do you guys think that I would have detonation problems with that high of a compression? I want to avoid running 100% race gas. Thanks

89Warbird

No you won't, depending on the cam choice will decide what octane you will have to run.

1bad91Z

iTrader: (1)

Those pistons you are talking about are the 4 valve relief forged Speed Pro's right?

I'm running similar pistons but mine are 2 valve relief (~ 10.5:1 compression) with my block deck height, 62cc vortec heads (my heads are milled to yeild 62cc chambers), and my head gaskets.

I run 93 octane pump gas just fine with no detonation.

So to answer your question, you can run pump gas with no problems, I would just strictly run 93 octane (super unleaded) to be safe.

AKL98

Thanks for your help guys, those pistons are the four valve relief speed pros and my cam is going to be a comp xtreme energy 268 degree with .480 max lift.

wallyZ/28

iTrader: (9)

my pistons are keith black hypo and they are 10:3 to 1 w/64cc..4v reliefs and the lt4 hot cam with comp pro mag 1.6 rockers......i only run reg unleaded in my car.no detonation or anything

Pony Killer

no worries about detonation with that low compression.

89Warbird

If you guys are listening for detonation then you won't hear it, the computer takes care of that. You have to hook up a scanner and watch to see if the ECM is pulling out spark lead due to the knock sensor detecting knock. I can tell you when I lug my engine below 2500 rpm my ECM is pulling out spark lead due to knock sensor picking up the start of detonation. After 2500 it stops pulling out spark lead, which means no detonation/knocking.

1Bad91Z, I bet you get a LOT of spark lead pulled out at 10.5:1

WallyZ28, same for you on regular gas. I run strictly 92 octane or better and like I said, I still see it.

I did a lot of research and calculating to figure out the max CR I could run with the Vortec heads and I pushed it right to the limit according to all the data logging I've done. To go higher you have to get more valve overlap to lower cylinder pressure or you have to go to ceramic coatings on the pistons and heads/valves. Or you have to run better gas.

That's not to say it can't be done or isn't being done, but I'd like to know how it's being done.

Captain C

I've had pinging with my car and it only has 9:1 compression.... I wish I could get 93 octane in my area, 91 is all we can get. (Other than a local Fastrip that has 100 octane unleaded....) I have to back my timing off to 10* in the summer or it will ping pretty badly. I'm getting really tired of replacing broken pistons..... (KB231's) I changed to TRW forged this time around...... But that's another horror story.....

A lot of my pinging may have to do with the chip, more may have to do with the poor chamber design of the heads. They are 76cc
World Products S/R Torquer (pocket ported) heads that were intended for a stock 400 originally

1bad91Z

iTrader: (1)

89Warbird -

I get no spark retard at all. I've been tuning my own PROM's for a while! I run 93 octane all the time and if you were to hook up my scanner (I use Datamaster and a laptop w/ ALDL cable), you would see that I'm correct. It's all about fuel maps and spark table control. You get the fuel and timing tables right and you can run 11:1 compression on 93 octane with-out the knock sensor killing your timing.

89Warbird

1bad91z - When you bog the car and lug it you don't get any spark retard? Wow, I would never have believed it. Maybe I need to do my own PROM. Right now Street and Performance are making my PROMS and I've been very happy with them.

1bad91Z

iTrader: (1)

Street and performance puts out the same crap as the other companies (TPIS, SLP, etc...) AND, they charge out the a$$ doing so!

They only way to get your PROM perfect, or as close to perfect as you can, is by doing it yourself. This involves alot of knowledge, datalogging, re-burning chips, etc..

That is the only way to do it. Every motor is different. Even if the specs of two motors are practicly identicle to the exact specification, they will still be different. There are too many external variables.

If you start out with a motor / tranny / wiring harness / ECM/ and all sensors in good mechanical shape, and you know what your doing when tuning your ECM, there's so much higher of a success rate on getting everything to run right and run with-out detonation or any other mechanical failures. Even with a radical small block with 11:1 compression, you can still run 93 octane pump gas and the stock ECM with a correctly custom tuned PROM can handle it just fine without experiencing knock retard, detonation, etc...